Hi Sergey,

How are things going? A bit retarded reaction from me - I was busy with some
other matters too (as also shown on the list). We can discuss this also
while sipping at a glas of beer, maybe..?
I do not dispute any of the problems/difficulties in relating isotopic
values of different materials (and different methods) with each other.

My concern is that none of the 'nicely propagated uncertainties' (as you
call it) actually is shown in any of the conversion equations (VSMOW-VPDB or
visa versa) as published... These equations all are presented as 'absolute'
conversions, what in fact is contradicted in both the stories by you and Ty.

Basically we do agree on most facts I think, but seem to have different
opinions about the fact if only one defined scale can exist (or if we need
sub-scales for practical use). Let me state clearly that I have nothing
against sub-scales, except that they may increase the uncertainty of the
measurement if that means extra treceability steps are also introduced by
them. Certainly relating a sub-scale to the international scale introduces
an uncertainty, because it is physically impossible to do that absolute.
Of course, it is possible to decrease uncertainty(precision) by not tracing
the measurements to the international scale (that in fact is the compromised
agreement by IUPAC commission to my opinion), and omitting the calibration
to VSMOW-SLAP (and therefore avoiding the extra uncertainty by decreasing
the number of 'traceability' steps). And of course, all following that same
procedure can compare with each other - so far no problem. That in fact is
what you like to do Sergey.

But now my question is how to proceed with applying the measured values, for
instance, as in many cases, with thermometry? Isotopic thermometers are
considered to be valid for isotopic data related to the international
standard (at least they should be! Or do I understand wrongly here???) - now
we should need two sets of thermometers in the case of two scales... Or else
nevertheless a large uncertainty has to be added to the thermometers if
using VPDB related measurements on thermometers related to the VSMOW-SLAP
scale. And to what extent we can compare these temperatures obtained by
different 'systems'? In fact they do not represent exactly the same
'temperatures scale' if we would follow this kind of procedures. While the
goal was to reduce uncertainty to have more 'sensitivity' in, for instance,
thermometrie.

Unfortunately, in too many cases no uncertainty related to the thermometers
(obtained generally empirically or theoretically, and have a relatively
large uncertainty) is included in applying these thermometers (so, for those
doing this also this problem does not exist). Repeating calculations with
the same equation - having a specific uncertainty - does not cancel out the
uncertainty (as might be thought by some) for the resulting temperatures!
Just some matters of practical sense...

I hope to have made more clear why I brought forward this matter again. We
simply can state that no ideal solution (far from) can be found at the
present stage of technology (you are very right there Ty).
But there simply cannot exist two defined scales. For example, having
defined values for d18O for the VSMOW and SLAP at one side and for NBS 219
at the other implies that it was meant to have such...; this is physically
incorrect and should be changed.
If having a sub-scale, used without relating to the VSMOW-SLAP scale, is not
incorrect if properly reported as such. However, if converting into
VSMOW-SLAP a considerable uncertainty (as clearly stated by Ty in fact - no
real certainty exist in relating the VPDB to VSMOW-SLAP!) should be included
in the the result than, unlike the 'absolute' conversion as given in
equations at present....

Best wishes,
Pier.

> From: <[log in to unmask]>
> Reply-To: Stable Isotope Geochemistry <[log in to unmask]>
> Date: Mon, 11 Dec 2006 19:23:26 +0100
> To: <[log in to unmask]>
> Conversation: [ISOGEOCHEM] On VSMOW - VPDB again
> Subject: Re: [ISOGEOCHEM] On VSMOW - VPDB again
> 
> Dear Pier and Ty,
> 
> I completely agree with Ty.
> The combined uncertainty of CO2 analyses, including the calibration to the
> primary CO2 standard in use or secondary standards (NIST CO2 gases - RM
> 8562-8563-8564) is much smaller than the combined uncertainty, when d18O(CO2)
> on the VPDB_CO2 scale converted to the VSMOW scale.
> In the latter case any inter-comparison of d18O(CO2) would be limited not by
> analytical capabilities but mostly by the scale conversion factor.
> 
> As long as many important measurements are made on CO2 gas (carbonates,
> monitoring of air CO2) and these practically do not relate to waters, I see
> that as a reason to report data on the VPDB_CO2 scale.
> In that case anyone can easily rescale values, with a nicely propagated
> uncertainty! 
> 
> Kind regards, 
> 
> Sergey
> 
> 
> ............................................
> Dr. Sergey Assonov
> Joint Research Centre (JRC) of European Commission (EC)
> Institute for Reference Materials and Measurements, Isotopic Measurements Unit
> Retieseweg 111, 2440 Geel, Belgium
> phone: +32-14-571623
> fax:     +32-14-571863
> ............................................
> 
>  
> 
> 
> ________________________________
> 
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
> Of Pier de Groot
> Sent: Monday, December 11, 2006 6:28 PM
> To: [log in to unmask]
> Subject: Re: [ISOGEOCHEM] On VSMOW - VPDB again
> 
> 
> Hi Ty,
> 
> If the conversion is so uncertain, why this is not shown by a large
> uncertainty in the conversion equations than???
> There are even more serious implications. If we keep the obsolete VPDB scale
> alive, then there are arguments for those working with other materials, such
> as organic components or atmospheric gases, also to have their own scales
> (proposals were already seen into that direction). If we do that, we
> completely loose any possibility ever to compare and will drift away from each
> other seriously in the end.
> 
> I very well realize the analytical problems of analyzing different materials
> with different methods (even same materials with different methods already can
> be tricky enough; new methods are introduced over time, as also will be in
> future, and actually make comparisons not easier), and the eventual
> uncertainty related to the calibration procedures. But the problem will be
> even bigger if we do not try to make a uniform scale for each isotopic ratio
> (how, for instance, 17O would fit into the 18O/16O scale is another problem,
> added to the already existing ones).
> 
> The decision in 1985 was probably a compromise one, while in metrology
> compromises never are a solution (to my humble opinion)....
> But considering the difficulty of the variety of sample materials and
> analytical methods, I can understand that (compromise) decision, but not
> necessarily I do need to agree with it, do I? I still think we should have a
> single scale, and if that introduces more uncertainty to measurements of some
> materials, we have to live with that fact. Now we have to live with another
> incorrect situation, that not cures any of the problems... Resuming, I can
> conclude that we agree on the fact that at this stage of our capability of
> analyzing we cannot reach an ideal situation or anything that comes close to
> that.
> 
> Thanks a lot for your answer to my posting as I asked to the community, I
> really appreciate that Ty.
> Nothing seems to be so complicated in analytical procedures as is metrology!
> And put two analysts together and you will get two opinions and two different
> outcomes of measurement (but hopefully not very far appart... i.e within the
> range of uncertainties for both).
> 
> Best wishes,
> Pier
> 
> 
> 
> 
> ________________________________
> 
> From: Tyler B Coplen <[log in to unmask]>
> Reply-To: Stable Isotope Geochemistry <[log in to unmask]>
> Date: Mon, 11 Dec 2006 10:57:03 -0500
> To: <[log in to unmask]>
> Subject: Re: [ISOGEOCHEM] On VSMOW - VPDB again
> 
> 
> Pier, 
> 
> The fact that we do not know accurately the conversion relationship between
> the VSMOW and VPDB delta O-18 scales is exactly the reason that the VPDB delta
> O-18 scale should be retaining.  It can be used in oceanography, for example,
> to easily express delta O-18 values of carbonates with the highest accuracy
> and precision.  This was recognized by IAEA consultants, meeting in Vienna in
> September of 1985, who anchored the VPDB delta O-18 scale by assigning a
> consensus value of -2.2 per mill exactly to NBS 19 calcium carbonate.
> 
> You are quite correct that it is important to normalize delta O-18 data.  VPDB
> delta O-18 data should be normalized so that if carbonates differing in delta
> O-18 values by -55.5 per mill were analyzed, their reported values would
> differ by -55.5 per mill.  This can be accomplished by interspersing NBS 18
> carbonate among unknowns and normalizing delta O-18 data by assigning a delta
> O-18 value on the VPDB scale to NBS 18 of -23.01 per mill (Coplen, 1996, More
> uncertainty than necessary, Paleoceanography, vol. 11, p. 369-370).
> 
> For any scale, authors are strongly encouraged to provide in every published
> report the delta values for internationally distributed isotopic reference
> materials had they been analyzed with their unknowns.  In this manner, their
> valuable data can be used most effectively by researchers in the future.
> 
> If analytical techniques improve sufficiently with new equipment and
> techniques, then it might be possible to go to one scale.  I look forward to
> that day. 
> 
> Sincerely, 
> 
> Ty 
> 
> 
> Pier de Groot <[log in to unmask]>
> Sent by: Stable Isotope Geochemistry <[log in to unmask]> 12/11/2006
> 06:52 AM 
> 
> Please respond to
> Stable Isotope Geochemistry <[log in to unmask]>
> 
> To 
> 
> [log in to unmask]
> 
> cc
> Subject 
> 
> On VSMOW - VPDB again
> 
> 
> 
> 
> Dear all,
> 
> I received questions about the conversion of VSMOW into VPDB (or visa versa)
> scale. It was noted that a while ago there was discussion already on this
> matter on the list, and Ty Coplen presented a corrected equation (caused by a
> better measurement of 'absolute' isotopic rations).
> 
> What still surprises me in this matter, where we do our best to get this
> conversion as good we can, that all equations are given as absolute
> conversions. This, unfortunately, gives the impressiuon that we have two
> defined absolute scales for O-isotope ratios - a situation that physically
> cannot exist. We only can have one defined scale (what to my knowledge is the
> VSMOW-SLAP scale), and all others must be related to that one, meaning
> introduction of uncertainty if converting to that 'secondary' scale (simple
> rule of traceability). Now, in all the equations published up to now this
> uncertainty is completely left out. There should be a plus/minus added with
> equations.
> 
> Even better would be to stop using completely this obsolete VPDB sub-scale for
> O-isotopes - it is just historical and only is continued out of sentiments and
> conservatism, and does not add anything else than confusion.
> It only introduces serious mistakes, as clearly shown by the conversion
> equations, missing the added uncertainty of this conversion....
> Is there any conclusion/decision on this important matter by the IUPAC
> commission? They should be aware of this wrong representation!
> 
> I really hope to get some reactions on this posting. It is an important
> subject that should be solved.
> I bring this up again because I think it still is not solved - it would be
> better if an organisation, like for instance IUPAC, is giving the correct
> directions in this matter. If these already would exist, I like to hear where
> to find it - then I must have overlooked one or two things!.
> 
> Best wishes,
> Pier.
> ****************************************************************
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